Poozie,
You'll need to spray your potatoes ASAP to prevent them getting infected. All is not lost even if you get some infection, pick off the diseased leaves and spray.

The only "organic" blight spray that I have used in the past has been the old "Burgundy Mixture" containing baking soda and blue stone (copper sulphate)
I prefer to use Dithane (Mancozeb) myself, the sachets are easily bought, mixed and it's a safe fungicide to use. You might need to spray every couple of weeks depending on weather conditions.

Is Bordeaux mixture 'organic' approved? I sprayed second earlies (anya) and some outdoor toms last week as i was convinced that i saw early signs on a couple of spuds. There has been a spotty leaf thing going on this year but this plant had rotting patches on the stem I dug it out -spuds not quite ready- I'm hoping to get away with it and i have not seen any advance since. Generally you can get the earlies etc. out the soil before any damage. Maincrops are more of a problem.

Paulottie,
Copper based sprays are approved by the Soil Association. However, there is some concern about the environmental impact of continued use of copper based sprays. As a result, organic farmers can now only apply a maximum of 6 kg Copper per hectare (about 5.5lbs per acre) per year. This equates to around 3 sprays per season. In a bad blight year, 3 sprays won't prevent or control blight.

Copper based sprays work much better later in the season to prevent tuber blight. If blight is already in the crop, they won't give control of the blight already present.

A 2% Bordeaux mix or even a 1% Burgundy mix could be used. I prefer the Burgundy mix as it's easier to mix and it doesn't clog up your sprayer like the Bordeaux mix can.

You're right that the problem is usually with the maincrop, in most years the earlies are harvested before the high blight risk.

Thanks for that jondanie. First time i've used it there in 4 years -so I think I'm within the limits! I'm not sure as I can remember the difference!-disgusting really as I used to work in Burgundy many years ago. I'll look it up as your right it did clog up even tho i mixed in a bucket and decanted it.

A recollection from my geology student days - if you're prospecting for copper in a jungly environment you look for clearings - the stuff doesn't grow well where there are copper minerals in the ground. They are actually called (or were - we're going back some!) 'copper clearings'. I can see why they limit the amount you can add to your soil.